Method and system for determining a position of an object using two-way ranging in a polystatic satellite configuration

ABSTRACT

A method and system for determining a position of an object utilizes two-way ranging and polystatic techniques. A first communication transceiver at a first known location provides a bidirectional communication path between the first communication transceiver and the object wherein the first communication transceiver transmits a first ranging signal to the object and the object transmits a second ranging signal to the first communication transceiver in response to the first ranging signal. The first communication transceiver further provides a first unidirectional communication path between the first communication transceiver and the object wherein the first communication transceiver performs one of transmitting a third ranging signal to the object and receiving a fourth ranging signal from the object. A second communication transceiver at a second known location provides a second unidirectional communication path between the second communication transceiver and the object wherein the second communication transceiver performs one of transmitting a third ranging signal to the object and receiving a fourth ranging signal from the object. A signal processor determines a first path length corresponding to a first time length of the bidirectional communication path, a second path length corresponding to a second time length of the first and second unidirectional communication paths, and the position of the object based on the first and second known locations and the first and second path lengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of copending U.S. applicationSer. No. 08/803,937 filed on Feb. 21, 1997. This application is relatedto U.S. application Ser. No. 08/803,936, filed Feb. 21, 1997, now U.S.Pat. No. 5,969,674, entitled “Method and System for Determining aPosition of a Target Vehicle Utilizing Two-Way Ranging” and is furtherrelated to co-pending U.S. application Ser. No. 08/803,935, filed Feb.21, 1997, entitled “Method And System For Determining A Position Of ATransceiver Unit Utilizing Two-Way Ranging in a Polystatic SatelliteConfiguration Including a Ground Radar.”

TECHNICAL FIELD

[0002] This invention relates to methods and systems for determining aposition of a transceiver unit, such as those provided on an airplane ora surface vehicle, utilizing two-way ranging through multiplesatellites.

BACKGROUND ART

[0003] Current Automatic Dependent Surveillance (ADS) technology, suchas Global Positioning System (GPS), Wide Area Augmentation System (WAAS)or GLONASS, provides positioning information utilizing satellitetransmissions. For example, the GPS, developed and deployed by the U.S.Department of Defense, consists of 24 satellites orbiting the earthtwice a day at an altitude of 12,000 miles, as well as five groundstations to monitor and manage the satellite constellation. Using atomicclocks and location data, GPS satellites transmit continuous time andposition information 24 hours a day to a GPS receiver, which listens tothree or more satellites at once to determine the user's position. Bymeasuring the time interval between the transmission and the receptionof a satellite signal, the GPS receiver calculates the distance betweenthe user and each satellite, and then uses the distance measurements ofat least three satellites to arrive at a position.

[0004] Such systems, however, utilize one-way ranging in which anaccurate, synchronized clock is required at each station. Anysynchronization error or error regarding the location of one of thesatellites results in an error in the determined position of the targetvehicle. Thus, there is a need to provide very accurate position andvelocity information with a high degree of integrity and reliability.

DISCLOSURE OF THE INVENTION

[0005] It is thus a general object of the present invention to provide amethod and system for determining a position of an object, such as anairplane or a surface vehicle, utilizing two-way ranging in a polystaticsatellite configuration to derive independent estimates of thetransceiver's state vectors including position and velocity.

[0006] In carrying out the above object and other objects, features, andadvantages of the present invention, a method is provided fordetermining a position of the object. The method includes the steps oftransmitting a first ranging signal from a first known location to theposition and transmitting a second ranging signal in response to thefirst ranging signal to the first known location. The method alsoincludes the steps of transmitting a third ranging signal from a secondknown location to the position and transmitting a fourth ranging signalto a third known location in response to the third ranging signal. Themethod further includes the step of determining a first delaycorresponding to a time difference between transmission of the firstranging signal and receipt of the second ranging signal. The method alsoincludes the step of determining a second delay corresponding to a timedifference between transmission of the third ranging signal and receiptof the fourth ranging signal. Finally, the method includes the step ofdetermining the position of the object based on the first, second, andthird known locations and the first and second delays.

[0007] In further carrying out the above object and other objects,features, and advantages of the present invention, a system is alsoprovided for carrying out the steps of the above described method. Thesystem includes a first communication transceiver at a first knownlocation for providing a bidirectional communication path between thefirst communication transceiver and the object wherein the firstcommunication transceiver transmits a first ranging signal to the objectand the object transmits a second ranging signal to the firstcommunication transceiver in response to the first ranging signal. Thefirst communication transceiver further provides a first unidirectionalcommunication path between the first communication transceiver and theobject wherein the first communication transceiver performs one oftransmitting a third ranging signal to the object and receiving a fourthranging signal from the object. The system also includes a secondcommunication transceiver at a second known location for providing asecond unidirectional communication path between the secondcommunication transceiver and the object wherein the secondcommunication transceiver performs one of transmitting a third rangingsignal to the object and receiving a fourth ranging signal from theobject. The system further includes a signal processor for determining afirst path length corresponding to a first time length of thebidirectional communication path, determining a second path lengthcorresponding to a second time length of the first and secondunidirectional communication paths, and determining the position of theobject based on the first and second known locations and the first andsecond path lengths.

[0008] The above object and other objects, features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a diagrammatic representation illustrating acommunication system employing the method and apparatus of the presentinvention;

[0010]FIG. 2 is a block diagram of the aircraft segment and the groundsegment included in the system shown in FIG. 1;

[0011]FIG. 3 is a block diagram of a preferred transmitter and apreferred receiver for the traffic controller station used in the systemof FIG. 1; and

[0012]FIG. 4 is a block diagram of a preferred transmitter and apreferred receiver for a vehicle in the system of FIG. 1.

BEST MODES FOR CARRYING OUT THE INVENTION

[0013] Referring first to FIG. 1, a communication system 10 with atypical geometry for practicing the present invention isdiagrammatically illustrated. In general, the system 10 includes atarget object 12, represented by an aircraft 12 in the preferredembodiment, although Earth-bound vehicles are also trackable with thepresent invention. A plurality of communication satellites 11 throughwhich aircraft 12 communicates with at least one traffic controllerstation 16 via a satellite ground station 14 are within the field ofview of aircraft 12 at a given time.

[0014] Communication satellites 11 are preferably in multiple planesusing Low Earth Orbit (LEO) satellite constellations and/or Medium EarthOrbit (MEO) satellite constellations such as Iridium, IntermediateCircular Orbit (ICO), Teladesic or Globalstar. In addition, aGeosynchronous Earth Orbit (GEO) satellite constellation may also beused in conjunction with the LEO and/or MEO satellite constellations.The planned ICO configuration with ten to twelve satellites in twoplanes is adequate to implement the position location and tracking ofaircraft 12 for navigation and landings (up to Category I) whileproviding the necessary global coverage.

[0015] Multiple dynamic communication links between aircraft 12 and asatellite ground station 14 are achieved via the plurality ofcommunication satellites 11, which are then utilized to deriveindependent estimates of the positions and velocities (state vectors) ofaircraft 12. To obtain more accuracy and flexibility, the presentinvention employs a polystatic configuration. A polystatic configurationconsists of several transceivers at separated locations, which cooperatewith each other. The transceivers may be stationary or moving.

[0016] In a monostatic configuration, the forward and return rangingsignals propagate through the same link. As such, the equal rangelocations of the measurement are confined to a spherical surfacecentered on the relay satellite position with a radius (range) equal toa distance between aircraft 12 and the relay satellite. By utilizingpolystatic techniques, in which the forward and return ranging signalspropagate through different satellites, the equal range locations of themeasurement are confined to an ellipsoidal surface. The two foci arelocated at the satellite positions so that the sum of the distancesbetween aircraft 12 and the two satellites 11 is a constant.

[0017] Satellite ground station 14, such as a Satellite Access Node(SAN), transmits a ranging signal to a targeted object, such as aircraft12, via one of communication satellites 11. Aircraft 12 then retransmitsa data message back down to ground station 14 via the same communicationsatellite 11 or a different one or set of communication satellites 11utilizing polystatic techniques. Preferably, traffic controller station16 informs the aircraft 12 of which return link strategy to employ priorto initiation of the two-way ranging. Each ranging signal transmitted bysatellite ground station 14 may be forwarded to the same satellite 11and then the retransmitted data messages from aircraft 12 may beforwarded through different satellites 11. Thus, the forward and returnranging signals can propagate through various links via differentsatellites, i.e., for each forward path, there are N return pathsavailable for a total of N×N possible links.

[0018] The positions in space of communication satellites 11 are knownso that corresponding ranges R₁, R₂, and R₃ between each ofcommunication satellites 11 and satellite ground station 14 are known.However, ranges R₁, R₂ and R₃ can be calibrated over time to obtain amore accurate measurement. The links R₄, R₅, and R₆ are then employed todetermine the state vectors by two-way ranging from satellite groundstation 14 to aircraft 12. The time difference between transmission ofthe ranging signal by the satellite ground station 14 and receipt by thesatellite ground station 14 of the responding ranging signal fromaircraft 12 is used in determining ranges R₄, R₅, and R₆.

[0019] In determining the position and velocity of aircraft 12, thepresent invention may be utilized in conjunction with GPS. When GPSsignals are available, the GPS signals are used to derive the aircraftstate vector which is then transmitted to traffic controller station 16,such as an Air Traffic Controller (ATC) facility, via communicationsatellites 11 and satellite ground station 14. Preferably, the ATCfacility 16 has signal processing capability. Alternatively, the signalprocessing capability may be located at satellite ground station 14.Simultaneously, ranging signals are sent by satellite ground station 14via communication satellites 11 to a targeted aircraft, such as aircraft12. Aircraft 12 then retransmits this ranging signal on a return linkback to satellite ground station 14. The returned two-way rangingsignals are detected and tracked in time and frequency by satelliteground station 14. Measured time and frequency values from multiplesatellite links are compared to predicted values. Improved estimation ofthe aircraft state vectors is accomplished through data fusion of thetwo independent measurements, i.e, the GPS measurement and the two-wayranging measurement. The updated aircraft state vectors are thentransmitted to aircraft 12.

[0020] The time stamps through various forward links arrive at aircraft12 in different time slots. It is possible to allow fixed processingdelays to multiplex the time stamps together, and then transmit themultiplexed ranging signal through different return links simultaneouslyor sequentially. However, it is also possible to transmit themultiplexed signal through a single return link to save return linkspace assets when needed. Similarly, the present invention is flexibleenough to save forward link assets also. In addition, it is possible touse ICO satellites either as forward or as return link relays (not both)and to utilize other (GEO, MEO or LEO) mobile satellites as thecomplementary link relays.

[0021] Turning now to FIG. 2 there is shown simplified block diagrams ofboth an aircraft segment 18 and a ground segment 20 of the presentinvention. Aircraft segment 18 includes a conventional GPS receiver 22for receiving GPS signals from a GPS satellite 24 via an antenna 25. GPSreceiver 22 sends a position signal to a conventional ExtendedKalman-Filter (EKF) 26 which tracks the position signal as a statevector. An optional input 27 to EKF 26 is a signal from an InertialNavigation System (INS), such as a conventional mechanical gyro systemwhich monitors the distance traveled by aircraft 12 from a predeterminedposition.

[0022] Aircraft 12 receives the ranging signals from communicationsatellites 11 via a second antenna 28. Second antenna 28 is preferably aretrodirective antenna implemented with a Butler matrix, a low-profiledigital beam former, and Wavelet-based Finite-Impulse-Response (WFIR)signal processing. The retrodirective antenna measures the direction ofthe received signal from communication satellite 11 and automaticallytransmits the return signal back to the same or a different one ofcommunication satellites 11. The Butler matrix implements a Fouriertransform forming a set of nearly orthogonal beams covering thefield-of-view and is a relatively inexpensive approach to realizing aretrodirective antenna. The low-profile digital beam former array lendsitself to a thin conformal array configuration which is preferred foraircraft installation. Optionally, a tracking antenna can be used inplace of the retrodirective antenna which consists of either anelectronically or mechanically steered antenna driven by a monopulse,step-scanned, or conically-scanned tracking loop.

[0023] In order to utilize polystatic techniques in the presentinvention, a digital implementation of the Butler matrix is alsopreferred, such as a conjugate gradient digital beam former, in order tomemorize the phase gradients of signals from various communicationsatellites 11, i.e, the direction of the incoming signals, and to applyproper phase conjugations to the outgoing signals so that the outgoingsignals are directed to the appropriate communication satellites 11.

[0024] The data between aircraft segment 18 and ground segment 20 can becombined with the unique ranging code signal in one of several ways: 1)Overlaying a Auslander-Barbano (AB) Code Division Multiple Access (CDMA)tracking code on the communication link channels as low-level AdditiveWhite Gaussian Noise (AWGN), thermal noise-like signals which slightlyraise the thermal noise floor; 2) Modulating the communication data withthe AB CDMA ranging code and sent as a single waveform, as shown in FIG.3; and 3) Separating the ranging links from data links. In the preferredembodiment shown in FIG. 3, ATC facility 16 transmits data which ismodulated by a WFIR waveform with a unique AB ranging code assigned toeach aircraft being tracked in the particular time slot. WFIR modulationenables the ranging signals to have variable resolution in addition tovariable length. The waveform specifically provides a means to transmita relatively wide-band WFIR ranging waveform over a group of narrow-bandcommunication satellite channels, simultaneously or sequentially, andsupports simultaneous ranging/doppler measurements and datademodulation.

[0025] The two-way ranging data 30 is sent to ground segment 20 viasatellite ground station 14. Two-way ranging data 30 is used to drive adual alpha-beta (α-β)/EKF tracking loop 32 wherein the fast α-β looptracks the AB CDMA code in communication coordinates, and the slow EKFtracks the target aircraft in Earth Centered Inertial (ECI) coordinatesto provide a unique preferred tracking architecture with low-complexity,high accuracy, and high integrity with fast-response valid-trackmetrics, and the ability to track out total-electron-content (TEC)induced waveform transmission range and doppler offsets.

[0026] The α-β loop is a relatively fast pair of time and frequencytracking loops which measure and smooth the received two-way rangingsignals during each access. The four-dimensional state vector Z for theα-β loop consists of the timing offset, time drift, frequency offset andfrequency drift. Time drift refers to clock drift whereas frequencyoffset refers to doppler shift due to link motion plus TEC. The statevector X for the EKF loop has 11 components consisting of thethree-dimensional ECI position coordinates, velocity, acceleration, andthe ranging plus doppler coordinates associated with ionospherical TECeffects.

[0027] Based on the α-β observation data from a previous access, the EFKloop predicts its state X_(k) at the state transition time k*T, where Tis the update interval for the EKF. This state is mapped into thecorresponding predicted state Z_(k) of the α-β loop. During the accessslot time ΔT, the α-β loop generates a smoothed state Z_(k) which isthen used by the EKF to smooth the predicted state to generate smoothedthe state X_(k). This allows the EKF to predict the state X_(k+1) at(k+1)*T. This procedure is repeated for the next access.

[0028] The predicted state vector from dual α-β/EKF tracking loop 32 andthe estimated state vector 34 from aircraft 12 are transmitted to aprocessor 36 which performs data fusion and validation between the twoindependent measurements to obtain an improved state vector estimation.Processor 36 also receives other terrestrial based data 37, such asposition of satellite ground station 14 and position of communicationsatellites 11. The improved state vector estimation is forwarded to ATCfacility 16 which then transmits this information to aircraft 12. Theimproved state vector estimation 38 received by aircraft 12 is processedby EKF 26 to generate a new state vector.

[0029] Referring now to FIG. 3, additional details of the receiver andtransmitter used in traffic controller station 16 are shown comprising atransmitter 40 and a receiver 42. Satellite ground station 14 transmitsdata which is modulated by a wavelet-based finite impulse response(WFIR) waveform with a unique AB ranging code assigned to each aircraft12 being tracked in the access time slot. The TDMA data to the targetedaircraft is modulated by the N-chip AB code sequence, unsampled by theWFIR sample rate M, and added with signals to other aircraft sharing thesame access slot. The summed output is filtered by a wideband WFIRfilter with overlaid envelope of the AB ranging waveforms. A bank ofnarrowband WFIR filters channelizes the wideband waveform into a set ofnarrowband waveforms which are compatible with the satellitecommunication channels such as ICO.

[0030] The receive processing at satellite ground station 14 is shown at42. The baseband signal from the digitizer, shown as ananalog-to-digital (A/D) function and an in-phase-quadrature (I/Q)function which may be combined is detected by a bank of narrowband (NB)WFIR filters matched to the ICO communication channels. The outputs areused to perform reconstruction of the wideband WFIR ranging signal foreach aircraft. This reconstructed wideband WFIR waveform is thendetected by on-time, early, and late correlators. The ranging time anddata from each aircraft is recovered by separate processing whichperforms the AB CDMA despreading, acquisition, tracking, time recovery,and data recovery.

[0031] As best shown in FIG. 4, aircraft receiver/transmitter 44preferably includes a retro-directive antenna 46. A Butler matrix, lowprofile digital beam form (DBF), and WFIR signal processing arepreferably employed. Retrodirective antenna 46 measures the direction ofthe received signal from satellite 11, and automatically transmits thereturn signal back to an appropriate satellite 11. The Butler matriximplements a Fourier transform forming a set of nearly orthogonal beamscovering the field of view, and has been proven to be a relativelyinexpensive approach to realize a retrodirective antenna. The lowprofile DBF array lends itself to a thin conformal array configurationwhich is preferred for aircraft installation. The implementationtechnique eliminates the need for an expensive tracking antenna on theaircraft which usually consists of either an electronically or amechanically steered antenna driven by a monopulse, step-scanned, orconically-scanned tracking loop.

[0032] The present invention works in many multiple-satelliteconstellations or combinations of multiple constellations. The presentinvention improves position and velocity accuracy in tracking a targettransceiver unit and provides a simple method to access more resourcesfrom space assets other than one constellation alone. Even if the GPS orGLONASS systems malfunction, the present invention still providesadequate position location and tracking measurements for global airtraffic control without complex clock and processing requirements.

[0033] While the best modes for carrying out the invention have beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

What is claimed is:
 1. A method for determining a position of an object,the method comprising: transmitting a first ranging signal from a firstsatellite at a first known location to the object as directed by asatellite ground station; transmitting a second ranging signal from theobject to the first satellite in response to the first ranging signalfor receipt by the satellite ground station; transmitting a thirdranging signal from a second satellite at a second known location to theobject as directed by the satellite ground station; transmitting afourth ranging signal from the object to the first satellite in responseto the third ranging signal for receipt by the satellite ground station;transmitting a fifth ranging signal from a third satellite at a thirdknown location to the object as directed by the satellite groundstation; transmitting a sixth ranging signal from the object to thefirst satellite in response to the fifth ranging signal for receipt bythe satellite ground station; determining a first delay corresponding toa time difference between transmission of the first ranging signal andreceipt of the second ranging signal; determining a second delaycorresponding to a time difference between transmission of the thirdranging signal and receipt of the fourth ranging signal; determining athird delay corresponding to the time difference between transmission ofthe fifth ranging signal and receipt of the sixth ranging signal; anddetermining the position of the object based on the first, second, andthird known locations and the first, second, and third delays.
 2. Themethod as recited in claim 1 wherein transmitting the first, third, andfifth ranging signals includes transmitting a ranging code relating tothe object.
 3. The method as recited in claim 1 wherein the object is anaircraft.
 4. The method as recited in claim 1 wherein the object is asurface vehicle.
 5. A system for determining a position of an object,the system comprising: a satellite ground station; a first satellite ata first known location, wherein the first satellite transmits a firstranging signal to the object as directed by the satellite ground stationand wherein the object transmits a second ranging signal to the firstsatellite in response to the first ranging signal for receipt by thesatellite ground station; a second satellite at a second known location,wherein the second satellite transmits a third ranging signal to theobject as directed by the satellite ground station, wherein the objecttransmits a fourth ranging signal in response to the third rangingsignal to the first satellite for receipt by the satellite groundstation; and a third satellite at a third known location, wherein thethird satellite transmits a fifth ranging signal to the object asdirected by the satellite ground station, wherein the object transmits asixth ranging signal to the first satellite in response to the fifthranging signal for receipt by the satellite ground station; wherein thesatellite ground station determines a first delay corresponding to atime difference between transmission of the first ranging signal andreceipt of the second ranging signal, a second delay corresponding to atime difference between transmission of the third ranging signal andreceipt of the fourth ranging signal, and a third delay corresponding toa time difference between transmission of the fifth ranging signal andreceipt of the sixth ranging signal, and determines the position of theobject based on the first, second, and third known locations and thefirst, second, and third time delays.
 6. The system as recited in claim5 further comprising a traffic control station in communication with thesatellite ground station, wherein the traffic control station includes asignal processor for determining the position of the object.
 7. Thesystem as recited in claim 5 wherein the first, third, and fifth rangingsignals include a ranging code relating to the object.
 8. The system asrecited in claim 5 wherein the object is an aircraft.
 9. The system asrecited in claim 5 wherein the object is a surface vehicle.